This argument makes no sense. Radeon Pro 500 series were also not "available" prior to the MBP 2017 refresh -- they were announced simultaneously. But I'm sure you understand that these chips had to have been available to Apple before this time? Availability or announcements of mobile graphics chips really means very little until they sit in an actual laptop, since these chips typically are not sold directly to consumers.
None of this indicates or counter indicates that MBP 2018 will have Vega Mobile chips, neither does the availability of the X chips really. I have not seen any indication anywhere that the Vega Mobile chips would be delayed. I do notice however, that AMD have been pretty timely with some other product launches, like the Ryzen 2000 series for example.
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They tend to refresh specs to current standards, for CPU and GPU at least. It's a bit unclear what might happen this year, since there are a few options for the CPU parts that Apple uses. It's reasonable to expect that all models get two more CPU cores. For GPU it's also unclear. AMD announced a Vega Mobile in Jan, but there's of course no word on whether it will make it into MBP 2018. It might and it mightn't. Beyond that there may well be other minor updates, probably nothing big.